---
title: "Stellar spectra from dark lines to abundance models"
description: "Learn stellar line motion, radiative transfer and fitting through three chapters, a calibrated-data tutorial and a 3D non-LTE paper guide."
canonical_url: "https://sandboxphysics.com/stories/stellar-spectroscopy/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/stellar-spectroscopy/"
---

AC03

# Stellar spectra from dark lines to abundance models

Sandbox Physics · 2026-10-28

One dark line carries several kinds of information. Start by separating motion from width, then follow the light through an emitting and absorbing layer. Fit a saved spectrum only after the instrument and continuum have joined the calculation.

[Move a stellar layer and change its lines →](https://sandboxphysics.com/labs/stellar-absorption-lines/)

[Start here What dark lines in starlight tell us Follow light through a stellar layer to separate the clues left by temperature, absorbing atoms, motion and a blurred spectrograph.](https://sandboxphysics.com/stories/stellar-spectroscopy/why-starlight-has-dark-lines/) [L1 Separate line position from line width Define vacuum wavelength, radial velocity and intensity, then derive a first Doppler estimate and calculate thermal and instrumental widths.](https://sandboxphysics.com/stories/stellar-spectroscopy/wavelength-motion-and-width/) [L2 Put absorption and emission in one equation Derive the uniform LTE slab solution, check transparent and isothermal limits, and connect optical depth to the curve of growth.](https://sandboxphysics.com/stories/stellar-spectroscopy/absorption-emission-and-growth/) [L3 From emergent intensity to a fitted spectrum Combine continuum placement, instrumental convolution and errors in one observing model, then test wavelength calibration and model mismatch.](https://sandboxphysics.com/stories/stellar-spectroscopy/from-intensity-to-a-fitted-spectrum/) [Tutorial Fit a frozen calibrated stellar spectrum Rerun pinned CALSPEC Vega data and a synthetic control with standard-library Python. Fit continuum, line centers and widths, then inspect failures.](https://sandboxphysics.com/stories/stellar-spectroscopy/fit-a-frozen-calibrated-spectrum/) [L4 · Paper guide Read a 3D non-LTE abundance paper Read a pinned iron-abundance paper figure by figure, checking model comparisons, correction signs, interpolation errors and stellar tests.](https://sandboxphysics.com/stories/stellar-spectroscopy/read-a-3d-non-lte-abundance-paper/)

The browser uses a stated teaching slab. The tutorial keeps observed CALSPEC fluxes, synthetic parameter-recovery data and fitted empirical profiles distinct. The final guide reads a published 3D non-LTE calculation.

## Sources and calculation scope

1. [Condon & Ransom, Essential Radio Astronomy, chapter 7.3](https://www.cv.nrao.edu/~sransom/web/Ch7.html)

   Equations 7.38–7.64: transition energy, Einstein detailed balance, stimulated emission as negative absorption, LTE populations and Planck source. Used to define the consistent artificial finite-level atom.

2. [STIS Data Handbook, section 4.3, Factors Limiting Flux and Wavelength Accuracy](https://hst-docs.stsci.edu/stisdhb/chapter-4-stis-error-sources/4-3-factors-limiting-flux-and-wavelength-accuracy)

   Wavelength zero point, dispersion and slit centering; used to distinguish an apparent profile displacement from a precision radial velocity.

3. [Krumholz, ASTR 4003/8003 (2019), Class 1, Radiative transfer I](https://www.mso.anu.edu.au/~krumholz/teaching/astr8003_s1_2019/notes1.pdf)

   Equations 24–28: integrating factor, formal solution and constant-source slab. Independently derived and checked by numerical transport here.

4. [Condon & Ransom, Essential Radio Astronomy, Appendix E](https://www.cv.nrao.edu/~sransom/web/A5.html)

   Transfer equation 2.27, LTE Kirchhoff relation 2.30 and Planck law 2.86; used at optical wavelengths with the full Planck function.

5. [STScI CALSPEC, alpha\_lyr\_stis\_011.fits (25 October 2022)](https://ssb.stsci.edu/trds/calspec/alpha_lyr_stis_011.fits)

   Pinned FITS header and all 82 selected observed STIS rows from 4750 to 4975 vacuum angstroms. File checksum and exact source rows accompany the tutorial.

6. [MAST reference atlases and CALSPEC releases](https://archive.stsci.edu/hlsp/reference-atlases)

   Data provenance and release context. The exercise deliberately pins version eleven rather than the changing current directory.

7. [Amarsi, Liljegren & Nissen (2022), 3D non-LTE iron abundances in FG-type dwarfs, arXiv:2209.13449v3](https://arxiv.org/pdf/2209.13449v3)

   Version three, 9 December 2022: sections 2.1–2.4 and 3.1, figures 2, 3, 8 and 10. The paper-reading guide checks conventions and sensitivity; it does not rerun the 3D solver.

8. [Amarsi et al., public non-LTE grids](https://zenodo.org/records/7088951)

   Author-linked model-data entry for later study; files are not loaded or reproduced by the browser Lab.